build: fix inability to detect correct python command in configure

The "which" utility is not guaranteed to be installed, and if it is, its
behavior is not portable.

Conversely, the "command -v" shell builtin is required to exist in all
POSIX 2008 compliant shells, and is thus guaranteed to work everywhere.

Examples of open-source shells likely to be installed as /bin/sh on
Linux, which implement the 12-year-old standard: ash, bash, busybox,
dash, ksh, mksh and zsh.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>

PR-URL: #32925
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

configure

@@ -5,11 +5,11 @@
 # as is the fact that the ] goes on a new line.
 _=[ 'exec' '/bin/sh' '-c' '''
 test ${FORCE_PYTHON2} && exec python2 "$0" "$@"  # workaround for gclient
-which python3.8 >/dev/null && exec python3.8 "$0" "$@"
-which python3.7 >/dev/null && exec python3.7 "$0" "$@"
-which python3.6 >/dev/null && exec python3.6 "$0" "$@"
-which python3.5 >/dev/null && exec python3.5 "$0" "$@"
-which python2.7 >/dev/null && exec python2.7 "$0" "$@"
+command -v python3.8 >/dev/null && exec python3.8 "$0" "$@"
+command -v python3.7 >/dev/null && exec python3.7 "$0" "$@"
+command -v python3.6 >/dev/null && exec python3.6 "$0" "$@"
+command -v python3.5 >/dev/null && exec python3.5 "$0" "$@"
+command -v python2.7 >/dev/null && exec python2.7 "$0" "$@"
 exec python "$0" "$@"
 ''' "$0" "$@"
 ]